The traditional way to make concrete in ready mix plants is to place sand, gravel and cement in separate hoppers feeding a drum or mixer which discharges into a concrete truck having a rotating drum to further mix en route to a job site where the cement is poured into the desired location, where it solidifies into concrete. Because the physical characteristics of each of the ingredients varies so much, traditional plants use powerful motors to mix, and the motors waste energy, and are inefficient. Typically, a batch will be twelve cubic yards of material premixed at the plant without water, and then placed in the concrete truck for further mixing by tumbling.
These traditional plants often do not have uniform mixing because of the large batch size and the differences in particle size among cement, fly ash, sand and aggregate. Water, if it is present, does not facilitate mixing, and sometimes hinders mixing when ingredients have hydrophobic surfaces. Also, the stiffness of the concrete affects the mixing characteristics. That is, high strength concrete has greater stiffness than low strength concrete, and the higher the strength, the more difficult it is to get complete mixing.
The strength of the concrete is measured by a variety of techniques, one of them is “slump,” an analog of the strength of the concrete. Slump is determined by taking a cone-shaped receptacle filled with mixed, but not set, concrete 16 inches deep, turning the receptacle upside down on a horizontal surface, and releasing the cone. As the cone slumps when unsupported by the receptacle, it indicates the stiffness of the concrete. A slump of four inches, for example, indicates a relatively weak concrete, while a slump of one inch indicates a relatively strong concrete. The strongest concrete has zero slump. It will be apparent that a concrete that does not slump does not mix as readily as one that inherently oozes around the ingredients to achieve better mixing.
If there is inadequate mixing, there are pockets of unmixed materials that will be either voids or non-adhering particles, either of which leads to a lower average strength. This has led designers of sky-scrapers, for example, to demand higher strength than would otherwise be called for in order to be sure the average strength meets the requirements. This over-engineering leads to unnecessary costs that could be avoided by thorough mixing.
The desire for adequate mixing is also reflected in the mixing cycles. If a plant normally has a 60 second mixing cycle for a 12 yard batch, the operator will run the plant at 60 seconds even with an 8 yard batch, just to be sure the mixing is adequate. This wastes energy and also leads to reduced throughput of the concrete plant with different sized batches.
Continuous mixing at concrete production plants can be stationary or mobile and can produce concrete very quickly. However, conventionally a skilled operator (i.e., batch operator) of the concrete production plant would be needed to manage the concrete production. Unfortunately, if the operator were not closely monitoring the concrete production, the processing can backup and overflow at one or more processing locations. Hence, there is a continuing need to enhance operation of continuous mixing concrete production plants.